Solid lead blocking sinks from behind (8)
I believe the answer is:
spheroid
'solid' is the definition.
(I've seen this before)
'lead blocking sinks from behind' is the wordplay.
'lead' becomes 'hero' (I've seen this in another clue).
'blocking' indicates putting letters inside (to block can mean to plug a gap in something).
'sinks' becomes 'dips' ('dip' can be a synonym of 'sink').
'from behind' says the letters should be written backwards.
'dips' written backwards gives 'spid'.
'hero' placed within 'spid' is 'SPHEROID'.
(Other definitions for spheroid that I've seen before include "Kind of globe, not perfectly round" , "Round shape" , "Solid" , "Almost-perfect-ball shape" , "Almost the shape of a ball" .)